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Scanning Electrochemical Microscopy: Principles and Applications to Biophysical Systems

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Scanning Electrochemical Microscopy: Principles and Applications to Biophysical Systems INSTITUTE OF PHYSICS PUBLISHING PHYSIOLOGICAL MEASUREMENT Physiol. Meas. 27 (2006) R63–R108 doi:10.1088/0967-3334/27/12/R01 TOPICAL REVIEW Scanning electrochemical microscopy: principles and applications to biophysical systems Martin A Edwards, Sophie Martin, Anna L Whitworth, Julie V Macpherson and Patrick R Unwin Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK E-mail: [email protected] Received 20 April 2006, accepted for publication 18 September 2006 Published 17 October 2006 Online at stacks.iop.org/PM/27/R63 Abstract This review highlights numerous and wide ranging biophysical and biochemical applications of scanning electrochemical microscopy (SECM). SECM instrumentation and theoretical modelling, necessary for experimental interpretation, are outlined, followed by a detailed discussion of the diverse applications of this technique. These include the measurement of flow through membranes, the determination of kinetic parameters of reactions, the investigation of the permeability of small molecules in tissues and monitoring biological processes, such as the production of oxygen or nitric oxide by cells. The significant impact of micro-electrochemical techniques on our understanding of basic physicochemical processes at biologically relevant interfaces is also considered. Studies reviewed include transport across and within bilayers and monolayers. Recent advances in SECM include the combination of SECM with other techniques, such as atomic force microscopy and optical microscopy. These developments are highlighted, along with prospects for the future. Keywords: scanning electrochemical microscopy, bilayers, monolayers, scanned probe microscopy, interfaces, electrochemistry, mass transport (Some figures in this article are in colour only in the electronic version) 1. Introduction Scanning electrochemical microscopy (SECM; the same acronym is used to describe the instrument) has developed into a powerful technique for quantitative investigations of interfacial physicochemical processes, in a wide variety of areas, as considered in several recent reviews (Barker et al 1999, 2001,Mirkin1999, Amemiya et al 2000, Mirkin and 0967-3334/06/120063+46$30.00 © 2006 IOP Publishing Ltd Printed in the UK R63 R64 Topical Review Figure 1. A selection of modes of operation of a SECM, illustrating how the current response, expressed as a normalized quantity (see the text), changes upon imaging certain features. Arrows represent the flow of the electroactive species (or ions) to the UME. Horrocks 2000, Yasukawa et al 2000b, Gyurcsanyia´ et al 2004,Puet al 2005). This review will provide a background to SECM, with particular reference to its use in characterizing biophysical processes and biomaterials. In the simplest terms, SECM involves the use of a mobile ultramicroelectrode (UME) probe, either amperometric or potentiometric, to investigate the activity and/or topography of an interface on a localized scale. The attractive features of SECM, for the study of biomaterials on a local scale, were recognized soon after the technique was formally established (Bard et al 1989, Kwak and Bard 1989a, 1989b). Early applications included quantitative studies of immobilized enzyme activity (Pierce et al 1992, Pierce and Bard 1993, Shiku et al 1995, 1996, 1997) and photosynthetic processes on leaves (Lee et al 1990, Tsionsky et al 1997b). These studies provided the foundations for the expansion into many new areas, as described in this review. These include the investigation of transfer processes, such as the passage of small molecules, e.g. oxygen, across biomimetic membranes (lipid bilayers and monolayers), and mapping the micron-scale porous nature of dentine or the sub-micron pores in membranes. Also included herein are applications that examine cellular activity and respiration and its variance with conditions (metastatic breast cancer cells, protoplasts and embryos), and studies of the permeability of oxygen in cartilage. SECM is able to resolve differences on the micron or sub-micron length scale, an advantage which is clear in the aforementioned examples and in many other biological situations. We begin with a basic overview of the principles and instrumentation for SECM, introduce the modelling techniques needed to understand the underlying processes and analyse experimental data, before considering specific methods and applications. We conclude with a brief overview of very recent developments and potential future developments, including hybrid techniques that involve SECM and the study of processes at the single-cell level. Several modes of SECM have been developed to allow the local chemical properties of interfaces to be investigated. A comprehensive review of all of the techniques can be found in Bard and Mirkin (2001). Figure 1 demonstrates the wide ranging information that can be extracted from the current of an amperometric UME, used as the probe in SECM. The arrows represent the flux of a redox-active species (or ion in the case of a microcapillary probe; see section 2.3.5 on micro-ITIES probes) to the UME. These examples will be developed further in section 4. Topical Review R65 (a) (b) Figure 2. Schematic views of (a) a rig, for SECM, and (b) a submarine electrode. 2. Instrumentation Although commercial instruments for SECM are available from several companies, including CH Instruments (USA), Windsor Scientific (UK) and Uniscan (UK), many instruments are still constructed by individual research groups; these are then tailored to specific applications. At the heart of SECM is the amperometric or potentiometric tip, whose location is controlled remotely, with appropriate positioners, relative to the sample interface. The type of experimental cell or vessel in which measurements can be made ranges from a simple beaker (Zhang et al 2000) to a Langmuir trough in a controlled atmosphere (Slevin et al 1998,Slevin and Unwin 2000). Electrochemical control and measurement in SECM is relatively simple as discussed in the next section. 2.1. Electrochemical instrumentation For amperometric control of the tip, with externally unbiased interfaces, a simple two-electrode system suffices (figure 2(a)). A potential is applied to the tip, with respect to a suitable reference electrode, to drive the process of interest at the tip and the corresponding current that flows is typically amplified by a current-to-voltage converter. If the sample is also to be biased externally, a bipotentiostat is required. For some studies of membrane transport, ion flow is driven from a donor to receptor compartment galvanostatically, and a potentiostatically controlled tip serves as a detector (Bath et al 2001). Potentiometric detection with UMEs R66 Topical Review of various types is readily accomplished (Amman 1986,Weiet al 1995), typically using a voltage follower with an input impedance appropriate to the type of indicator electrode used. 2.2. Positioning The tip is attached to positioners, which allow it to be moved and positioned relative to the interface under investigation. A variety of positioners have been employed in SECM instruments, with the choice depending on the type of measurement and spatial resolution required. For the highest (nanometre) resolution, piezoelectric positioners similar to those used in scanning tunnelling microscopy (STM) are mandatory (Liu et al 1986). There has also been some use of stepper motors to control the position of the tip in the x–y plane (Kranz et al 1995a, 1995b,Hlivaet al 1998), parallel to the interface of interest. In the application of SECM at solid/liquid interfaces (section 4.1), high-resolution x, y, z positioning and scanning is usually required. However, many SECM measurements, e.g. at air/liquid interfaces (section 6.1.2), simply involve the translation of a tip towards and/or away from a specific spot on an interface, in the perpendicular (z) direction. In this situation, it is only necessary to have high-resolution z-control of the tip, typically using a piezoelectric positioner, while manual stages suffice for the other two axes (Slevin et al 1996,Barkeret al 1998). It has further been shown that SECM measurements can be made with manual stages on all axes, with the z-axes driven by a differential micrometer and the x–y stages controlled by fine adjustment screws. This simple cost-effective set-up allows tip approach measurements to be made with a spatial resolution of ±0.25 µm (Martin and Unwin 1997, 1998b). The use of a video microscope, aligned such that the electrode may be observed from the side, has proved useful in facilitating the positioning of the UME probe relative to the interface of interest (Slevin et al 1996,Barkeret al 1998). 2.3. Probes The type of probe electrode used in SECM depends on the particular process under investigation. A diversity of probes is available for amperometry and potentiometry. Since these often have to be prepared in house, we highlight some of the most important tip designs in this section. In-depth reviews of UME design, fabrication and characterization can be found in Zoski (2002) and Forster (2003). 2.3.1. Micron-sized disc-shaped electrodes sealed in glass. Typically, amperometry involves electrolysis at a solid UME, usually a disc-shaped electrode, with a diameter of 0.6–25 µm. This type of electrode is readily fabricated by sealing a wire of the material of interest, in a glass capillary, making an electrical connection and polishing the end flat; see figures 3(a) and (b) for illustrations of such an electrode (Bard et al 1989, 1993, Wightman and Wipf 1989). Pt, Au and C electrodes have been successfully fabricated in this way. For most SECM studies, the ratio of the diameter of the tip (electrode plus surrounding insulator, 2rs) to that of the electrode itself, 2a,RG= rs/a is typically around 10. This minimizes effects from back diffusion (from behind the probe), making the electrode response more sensitive to the surface process. SECM images may be convoluted with both activity and topographical contributions. To resolve such effects, it may be possible to scan the sample twice, with the mediator of interest and then with a moiety that is inert with respect to the sample, so mapping the topography (Gonsalves et al 2000a, 2000b).
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